1. Field
This disclosure relates generally to a communication system and, more specifically, to techniques for reducing interference in a communication system.
2. Related Art
A wide variety of electronic devices have employed variable gain amplifiers (VGAs). A segmented variable gain amplifier (SVGA) is a type of VGA that has been employed in conjunction with power amplifiers (PAs) in transmitters of various devices, such as subscriber stations (SSs) in communication systems. The implementation of SVGAs in complementary metal-oxide semiconductor (CMOS) devices is a relatively new approach for mobile telephone transceiver designs. In general, employing an SVGA in a transmitter of a communication system may facilitate an increase in dynamic range and transmit power change accuracy. Unfortunately, stepping SVGA gain operation may cause radio frequency (RF) glitches at an output of an associated transmitter. The RF glitches may degrade system performance by introducing noise in adjacent/alternate channels. Transient adjacent/alternate channel leakage ratio (ACLR) is a measure of gain switching effects on spectrum purity in adjacent channels (at 5 MHz in wideband code division multiple access (WCDMA) systems) and alternate channels (at 10 MHz in WCDMA systems). In WCDMA systems, transient ACLR is defined as the ratio between average power and peak instantaneous power at an offset of 5 MHz and 10 MHz from an assigned carrier. WCDMA signals of user equipment (UE) typically have a peak-to-average power ratio (PAR) of 3 to 7 decibel (dB) at a transmit antenna. Depending on an operating antenna power level, an SVGA gain adjustment (which may be rapidly implemented) can lead to failure of transient ACLR specifications in an adjacent channel (5 MHz) and/or in an alternate channel (10 MHz).
Various conventional design approaches have addressed RF glitch problems associated with power amplifier (PA) gain switch. In one conventional design approach, digital gain step at baseband is employed to offset transitions in RF power. In this case, RF glitches are minimized by optimizing timing between RF gain adjustment and baseband gain adjustment. In general, when RF glitch duration (caused by PA gain adjustment) ranges from about 4 to 20 microseconds, prior art approaches adequately meet transient ACLR specifications. However, the prior art approaches are not generally adequate in devices that employ fast gain stepping (e.g., in situations where gain adjustment (switching) times are in the nanoseconds). For example, timing alignment becomes difficult and at times impossible depending on how much baseband poles vary with process and temperature. Moreover, conventional approaches have not taken into account that transient ACLR performance is data and modulation dependent and varies with measurement period.